698 



Mr. Ernest Clarke 



[June 9y 



the ciliary muscle acts it squeezes the periphery and causes the centre 

 to ])ulge, and so the increased convexity is limited to the centre, and 

 the aberration that would result is prevented by the contraction of 

 the pupil that always normally accompanies the act of accommoda- 

 tion, just as in photography we stop down a lens when focussing for 

 near objects. I have known this " squeezability " of the lens {if I 

 may use the term) in a child so great as to amount to 20 D. It 

 gradually decreases with age. At the age of ten the average 

 accommodative power is about 14 or 15 D; whereas, at the age of 



Near 

 Point. 



Age. 

 /O '5 20 25 30 5S 40 46 SO SS €0 6S 70 7S 8a 



IC ... 



14 ... 



20 ... 



25 ... 



40 ... 



Fig. 1. 



thirty, it has been reduced to 7 D, i.e. half (see Fig. 1). This loss of 

 elasticity is due to the sclerosing or hardening process which takes 

 place in the lens, as in all the tissues of the body, as age advances, 

 but it is not recognized by the individual until middle life, in the 

 normal about the age of forty-five or forty-eight, and then the 

 accommodation has sufficiently failed to necessitate the use of convex 

 glasses for near work, and this, mark you, not through any loss of 

 muscle-power, Imt through the hardening of the lens. 



Perhaps Donders' best work was on the subject of presbyopia. 

 His well-known diagram (Fig. 1), showing the loss of accommodation 



